Well, then.  I do not think our gravel-pit will tell us what we want:  but I know one which will.

You have all heard of Lady Grenville’s lovely place, Dropmore, beyond Maidenhead; where the taste of that good and great man, the late Lord Grenville, converted into a paradise of landscape-gardening art a barren common, full of clay and gravel-pits.  Lord Grenville wanted stones for rockwork; in those pits he found some blocks, of the same substance as those of Stonehenge or Pirbright.  And they contain the answer.  The upper surface of most of them is the usual clear sugar-sandstone:  but the under surface of many has round pebbles imbedded in it, looking just like plums in a pudding; the smaller above and the larger below, as if they had sunk slowly through the fluid sand, before the whole mass froze, as it were, suddenly together.  And these pebbles are nothing else than rolled chalk flints.

That settles the matter.  The pebbles could not come from Wales; there are no flints there.  They could not have been made before the chalk; for out of the chalk they came; and the only explanation which is left to us, I believe, is, that over the tops of the chalk downs; over our heads where we stand now, there once stretched layers of sand and gravel, “Tertiary strata” as I have been calling them to you; and among them layers of this same hard sandstone.

When the floods came they must have swept away all these soft sands and gravels (possibly to make the Bagshot sands, of which I shall speak presently), and left the chalk downs bare; but while they had strength to move the finer particles, they had not generally strength to move these sandstone blocks, but let them drop through, and remain upon the freshly-bared floor of chalk, as the only relics of a tertiary land long since swept away; while some were carried off, possibly by icebergs, as far as Pirbright, and dropped, as the icebergs melted, both there, at Dogmersfield, and also, though few and small, in Eversley and the neighbourhood.

But how came these tertiary sandstones to be so very hard, while the strata around them are so soft?

Ladies and gentlemen, I know no more than you.  Experience seems to say that stone will not harden into that sugary crystalline state, save under the influence of great heat:  but I do not know how the heat should have got to that layer in particular.  Possibly there may have been eruptions of steam, of boiling water holding silex (flint) in solution—­a very rare occurrence:  but something similar is still going on in the famous Geysers or boiling springs of Iceland.  However, I have no proof that this was the cause.  I suppose we shall find out some day how it happened; for we must never despair of finding out anything which depends on facts.

Part of the town of Odiham, and of North Warnborough, stands, I believe, upon these lower beds, which are called by geologists the Woolwich and Reading beds, and the Plastic clays, from the good brick earth which is so often found among them.  But as soon as you get to Hook Common, and to Dogmersfield Park, you enter on a fresh deposit; the great bed of the London clay.